The present invention relates generally to ultraviolet lamp systems and, more particularly, to microwave-excited ultraviolet lamp systems configured to irradiate a substrate with ultraviolet radiation.
Ultraviolet lamp systems are commonly used for heating and curing materials such as adhesives, sealants, inks, and coatings. Certain ultraviolet lamp systems have electrodeless light sources and operate by exciting an electrodeless plasma lamp with either radiofrequency energy or microwave energy. In an electrodeless ultraviolet lamp system that relies upon excitation with microwave energy, the electrodeless plasma lamp is mounted within a metallic microwave cavity or chamber. One or more microwave generators are coupled via waveguides with the interior of the microwave chamber. The microwave generators supply microwave energy to initiate and sustain a plasma from a gas mixture enclosed in the plasma lamp. The plasma emits a characteristic spectrum of electromagnetic radiation strongly weighted with spectral lines or photons having ultraviolet and infrared wavelengths. To irradiate a substrate, the radiation is directed from the microwave chamber through a chamber outlet to an external location. The chamber outlet is capable of blocking emission of microwave energy but allows electromagnetic radiation to be transmitted outside the microwave chamber. A fine-meshed metal screen covers the chamber outlet of many conventional ultraviolet lamp systems. The openings in the metal screen transmit electromagnetic radiation for irradiating a substrate positioned outside the microwave chamber, yet substantially block the emission of microwave energy.
The electrodeless plasma lamp emits a characteristic spectrum isotropically outward along its cylindrical length. Part of the emitted radiation moves directly from the plasma lamp toward the substrate without reflection. However, a significant portion of the emitted radiation must undergo one or more reflections to reach the substrate. To capture this indirect radiation, a reflector can be provided that is mounted within the microwave chamber in which the plasma lamp is positioned. The reflector includes surfaces capable of redirecting incident radiation in a predetermined pattern toward the chamber outlet and to the substrate positioned outside the microwave chamber.
A major shortcoming of conventional systems is the inability to accurately predict the focal point or focal plane outside the microwave chamber at which the reflected ultraviolet radiation will be delivered. Another shortcoming is the reflector of the lamp system cannot be easily modified to adjust the focal point or focal plane, if known, so that the substrate can be repositioned relative to the lamp system. Further, the inability to accurately predict the focal point or focal plane limits the ability to mass produce lamp systems capable of delivering predictable radiation patterns to a substrate. A further limitation is that conventional ultraviolet lamp systems are designed to irradiate a flat surface on large-area substrates and cannot be easily adapted to uniformly irradiate substrates in a surrounding fashion. For example, conventional ultraviolet lamp systems cannot uniformly irradiate the entire circumference of round substrates.
If the plasma lamp is considered a line source of radiation, the intensity of ultraviolet radiation striking the substrate is inversely proportional to the separation between the plasma lamp and the substrate. As a result, the ultraviolet radiation is significantly attenuated when traveling from the plasma lamp on the interior of the microwave chamber to the substrate positioned outside the microwave chamber. To compensate for this loss in intensity, the microwave power must be elevated to increase the output of the plasma lamp. However, the amount of infrared radiation will likewise increase with the output of the plasma lamp. The excess infrared energy heats the substrate, the microwave chamber, and the plasma lamp. The elevation in temperature associated with the excess infrared energy can significantly reduce the lifetime of the plasma lamp and can produce additional undesirable effects.
Thus, a microwave-excited ultraviolet lamp system is needed with a configuration capable of uniformly irradiating a substrate positioned within the microwave chamber with ultraviolet radiation and that can do so without emitting significant amounts of microwave energy.
The present invention overcomes the foregoing and other deficiencies of conventional microwave-excited ultraviolet lamp systems. While the invention will be described in connection with certain embodiments, the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention.
According to the present invention, an ultraviolet radiation generating system for treating a coating on a substrate, such as a coating on a cable or, more specifically, a coating on a fiber optic cable, comprises a microwave chamber having an inlet port capable of permitting the cable to be positioned within or to travel within a processing space of the microwave chamber. During operation, the microwave chamber is substantially closed to emission of microwave energy and the emission of ultraviolet radiation. A microwave generator is coupled to the microwave chamber for exciting a longitudinally-extending plasma lamp mounted within the processing space of the microwave chamber. The plasma lamp emits ultraviolet radiation for irradiating the substrate. A first portion of the ultraviolet radiation directly irradiates the frontside of the substrate. Mounted within the microwave chamber is a pair of reflectors which substantially surround the processing space. The reflectors are capable of reflecting a portion of the ultraviolet radiation for indirectly irradiating the backside of the substrate with reflected ultraviolet radiation.
In certain embodiments, the microwave chamber may further include an outlet port so that the substrate travels between the inlet and outlet ports through the microwave chamber at least partially within the processing space. In other embodiments, the lamp system may also include an ultraviolet-transmissive conduit positioned within the microwave chamber generally between the inlet and outlet ports. The conduit encloses the substrate when it is positioned within the processing space of the microwave chamber. In still other embodiments, the lamp system may also include microwave chokes which are capable of reducing the emission of microwave energy from the inlet and outlet ports.
According to methods of the present invention, a substrate is positionable within a processing space of a microwave and a plasma lamp is excited with microwave energy to emit ultraviolet radiation for irradiating the substrate. While the substrate is positioned within or traveling through the processing space, the frontside of the substrate is irradiated with direct ultraviolet radiation emitted from the plasma lamp and the backside of the substrate is irradiated with indirect ultraviolet radiation emanating from the plasma lamp which is reflected from a pair of reflectors. The substrate is removed from the processing space after irradiating.
The present invention permits the substrate to be positioned directly within the microwave chamber for treatment with ultraviolet radiation. As a result, the chamber may be completely sealed to prohibit the emission of microwave energy and to eliminate the need to emit ultraviolet radiation from the microwave chamber. Because the substrate, the plasma lamp, and the reflector have well-defined relative positions within the microwave chamber, the plasma lamp and reflector can be precisely located relative to the substrate for purposes of providing a predictable, reproducible and substantially uniform pattern of radiation at and distributed about or surrounding the substrate. Furthermore, because the substrate is positioned within the microwave chamber and because the ultraviolet radiation does not have to be transmitted through a screen to a location outside of the microwave chamber, a greater intensity of ultraviolet radiation per unit measure of microwave energy can be delivered to the substrate. As a result, the microwave energy can be reduced to deliver a given intensity of ultraviolet radiation to the substrate or the ultraviolet intensity can be optimized for improving the treatment throughput of the lamp system.
The above and other advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.